This invention relates generally to high capacity mobile communications systems, and more particularly to digital transport of radio frequency signals in a microcellular communication system.
A conventional cellular phone system 5 is shown in FIG 1A. Such systems are currently in widespread use in the United States. As illustrated in FIG 1A, system 5 has a fixed number of channel sets distributed among the base stations 12, 13 serving a plurality of cells 11, 16 arranged in a predetermined reusable pattern. Typical cell areas range from 1 to 300 square miles. The larger cells typically cover rural areas and smaller cells cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
A mobile unit 10 in a cell 11 has radio telephone transceiver equipment which communicates with similar equipment in base stationisites 12, 13 as the unit moves from cell to cell. Each base station 12, 13 relays telephone signals between mobile units 10 and a mobile telecommunications switching office (MTSO) 17 by way of communication lines 18. The lines 18 between a cell site, and the MTSO 17, typically T1 lines, carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions. The MTSO 17 is also connected through paths 19 to a switched telephone network 15 including fixed subscriber telephone stations as well as various telephone switching offices.
MTSO 17 in FIG 1A includes a switching network for establishing call connections between the public switched telephone network 15 and mobile units 10 located in cell sites 11, 16, and for switching call connections from one cell site to another. In addition, the MTSO 17 includes a dual access feeder for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art and utilize features such as phase ranging to indicate the distance of a mobile unit from a receiving cell site, triangulation, and received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO 17 is a central processing unit for processing data received from the cell sites and supervisory signals obtained from the network 15 to control the operation of setting up and taking down call connections.
A conventional base station 12 is illustrated in FIG. 1B. A radio controller unit 22 provides the interface between the T1 lines from the MTSO and the base station radio equipment. Transmitters 23, one for each channel serviced by the base station, are driven by circuit 22, which supplies each transmitter with an analog voice signal. Next, the signals are passed to a separate nonlinear power amplifier for each channel, or the signals may be combined and applied to a single linear power amplifier 24 as shown in FIG 1B. The output of power amplifier 24 is applied through duplexer 25 to antenna 26, to be broadcast into the cellular area serviced by the base station.
Signals received in antenna 26 are applied through duplexer 25 to multi-coupler 27. Multi-coupler 27 applies the wideband signal to receivers 28 (one for each channel), and scanning receiver 28b. The analog voice signal outputs of receivers 28 are applied to circuit 22. Base station 20 may optionally include a diversity antenna 26xe2x80x2, corresponding diversity filter 25xe2x80x2, multi-coupler 27xe2x80x2, and a plurality of diversity receivers 28xe2x80x2, one for each associated main receiver 28. Where implemented, the outputs of diversity receivers 28xe2x80x2 are applied to circuit 22, which would thus include circuitry for selecting the strongest signal as between corresponding receivers 28 and 28xe2x80x2 using known techniques. Scanning receiver 28b monitors the strength of signals in neighboring cells to identify mobiles which are potential candidates for being handed into its own cell.
In densely populated urban areas, the capacity of a conventional system 5 is severely limited by the relatively small number of channels available in each cell 11, 16. Moreover, the coverage of urban cellular phone systems is limited by blockage, attenuation and shadowing of the RF signals by high rises and other structures. This can also be a problem with respect to suburban office buildings and complexes.
To increase capacity and coverage, a cell area can be subdivided and assigned frequencies reused in closer proximities at lower power levels. Subdivision can be accomplished by dividing the geographic territory of a cell, or for example by assigning cells to buildings or floors within a building. While such xe2x80x9cmicrocellxe2x80x9d systems are a viable solution to capacity and coverage problems, it can be difficult to find space at a reasonable cost to install conventional base station equipment in each microcell, especially in densely populated urban areas. Furthermore, maintaining a large number of base stations spread throughout a densely populated urban area can be time consuming and uneconomical.
ATandT has proposed a system to solve the problem of coverage in urban areas without having to deploy a large number of conventional base stations. The system is shown and described with respect to FIG. 1 of ATandT""s European Patent Application No. 0 391 597, published on Oct. 10, 1990. A system of the type described in the ATandT application is shown in FIG. 1C of the present application. In the system of FIG. 1C, a grid of antenna sites 40 is placed throughout the microcellular system. An optical fiber network 42 interconnects the antennas with the base station 44. The base station 44 is connected to a mobile telecommunications switching office (MTSO) 52 by way of communication lines 54. Optical wavelength carriers are analog modulated with RF mobile radio channels for transmission through the optical fiber network 42 to the antenna sites 40. A detector circuit is provided for each antenna site 40 to receive the modulated carrier and reconstruct an RF signal to be applied to the antenna sites 40 for transmission into the microcell area 50. RF signals received at antenna sites 40 from mobile units are likewise modulated onto a fiber and transmitted back through optical fiber network 42 to base station 44. All of the channels transmitted from base station 44 are distributed to all antenna sites 40. Also, all the channels transmitted from the base station 44 can be received from the mobile units in any microcell area 50 and transmitted via optical fiber base station 44.
The above-described system of FIG. 1C has certain limitations. The ability to analog modulate and demodulate light, the limitations imposed by line reflections, and path loss on the fiber all introduce significant distortion and errors into an analog modulated signal and therefore limit the dynamic range of the signals which can be effectively carried via an analog system, especially in the uplink direction. These factors limit the distance from the base station to the antenna sites.
Moreover, in amplitude modulated optical systems an out-of-band signal is required to transmit control and alarm information to and from the antenna sites, again adding to the expense of the modulation and demodulation equipment.
The problems associated with analog optical modulation are addressed in a system described in commonly assigned U.S. patent application Ser. No. 08/204,660, now U.S. Pat. No. 5,627,879. In this system, a composite RF signal occupying the entire 12.5 MHz cellular band is sampled and digitized by a wideband digitizer at a rate greater than twice the bandwidth of the composite signal (in this case the sample rate is 30.72 MHz). The digitized signal is transported serially at 552.96 Mbps to a mote site over an optical fiber using digital optical modulation. The digitized signal is then converted back into a replica of the composite RF signal at the remote site, and amplified for retransmission. The reverse path composite signal is digitized and transported similarly in the opposite direction over the same fiber using wavelength division multiplexing. Because the optical modulation is digital, there is no loss of dynamic range over the fiber.
Both the ATandT analog system and the wideband digital system (described in the previous paragraph) teach the use of dedicated fiber lines installed for each remote antenna site. Alternatives to dedicated fiber lines, including existing telephone circuits, are typically designed to carry data streams much slower than 552.96 Mbps and the installation of additional or higher bandwidth fiber systems is an expensive and time consuming undertaking.
The data transferred over the dedicated fiber lines could be compressed by demodulating each channel, transmitting the demodulated signal, and remodulating the signals at the receiving end. Such a system would achieve transmission in the limited bandwidth, however, such a compression would not be transparent in the system, since a scanning receiver would be deprived of the necessary signals to perform hand in.
There is a need in the art for a cellular communication system which provides a scanning receiver channel data and hand in data using limited bandwidth transmission lines. The cellular communication system should employ existing telephone circuits and provide the scanning receiver the necessary hand in information without an excessive burden on the transport bit rate and without adding unnecessary complexity to the system hardware. The cellular communication system should also provide transparent operation of the existing scanning receiver to prevent redundancy in receiver hardware.
According to one exemplary embodiment of the present invention, there is provided a microcell system wherein a plurality of commonly located microcell base station units communicate with a corresponding plurality of microcell antenna units deployed in respective microcell areas. Each base station unit includes conventional RF base station transmitter and receiver pairs, one for each channel assigned to the microcell. Additional receivers are also provided to receive diversity channels, and to scan channels from neighboring cells. The RF signal outputs from the transmitters are combined and applied to a broadband analog-to-digital converter. Each microcell unit receives a digitized RF signal and reconstructs the analog RF signal using a digital-to-analog converter. The reconstructed RF signal is applied to a power amplifier, the output of which is fed to an antenna for broadcast into the microcell area.
The antenna units include both a main and a diversity antenna. The antennas each independently receive RF signals from the mobile units. The RF signal from the main antenna is applied to an analog-to-digital converter. A second filter receives the diversity signal from the diversity antenna, and applies that signal to a second analog-to-digital converter. The digitized representations of the main and diversity signals can be transported from the remote location to the base station via a high speed digital fiber path.
Thus, the exemplary embodiment outlined above contemplates that the microcell base station/antenna unit pairs are arranged to provide a reusable pattern of channels (as in conventional cellular technology) in the microcell system. The microcell base station units do not normally include an antenna, and can be located in a convenient and preferably low cost location, which may be outside of the microcell system territory if desired.
In one embodiment, digital signal processing is used to reduce the bit rate, so that the signals can be transported from the remote location to the base station via the telephone network, rather than a dedicated fiber. In particular, one embodiment uses a T1 interface between the microcells and the base station. The base station includes a scanning receiver for cell operations, such as hand-in. The scanning receiver analyzes received signal strength indication level (RSSI level) information and generates commands based on the information. Some scanning receivers monitor RSSI levels of adjacent cell clusters and the supervisory audio tone signalling (SAT signalling) information. Since the T1 line lacks the bandwidth needed to transport all baseband voice and control signals for all neighboring microcells, a system has been demonstrated which transports only the necessary information for hand-in evaluation, and then reconstructs the wideband signal in order to use existing scanning receivers at the base station.
In one embodiment, a signal processor at each microcell (or more generally, xe2x80x9cremote unitxe2x80x9d) tabularizes and transmits RSSI level and SAT frequency information of neighboring cells to the base station. The base station decodes this information and performs frequency domain to time domain conversion and reconstructs a wideband representation of signals from neighboring cells at their corresponding signal level, and with their corresponding SAT modulation. The baseband signals of the particular microcell and the control signals of all cells of interest are thereby added to a composite signal for digital to analog conversion. The analog signal is thereby presented to the scanning receiver for hand, in processing.
In one embodiment, a complex mixing is performed and a Hilbert Transform Filter is used to generate real outputs. In another embodiment, a positive frequency input to an inverse Fast Fourier Transform module provides real outputs without requiring a Hilbert transform.
In another embodiment, a system featuring frequency modulation of the SAT tones to each of the carriers is described. In one embodiment, fixed frequency offsets are provided to improve accuracy of center frequencies without increasing the size of the inverse fast Fourier transform used in the system.